We propose to investigate several current hypotheses concerning the mechanisms of exercise hyperemia in skeletal muscle using criteria not employed in previous investigations. This will be done by determining whether or not the direct correspondence between vascular resistance and the level of PO sub 2, oxygen consumption, K ion, and osmolarity seen in steady state exercise also exists in a wide range of time varying conditions. The time behavior of changes in each of these variables will be assessed during sinusoidal fluctuations in muscle contraction rate. We will evaluate each hypothesis on the basis of whether changes in the level of the suspected mediator coincide in time with changes in vascular resistance. This dynamic study is based on the control theory technique of frequency analysis.